Physiological fundamentals of entrapped microbial cells in wastewater treatment applications

There have been numerous studies on different applications of entrapped cells for wastewater treatment but none of them sought to understand the fundamentals on whether and how the entrapped cells function differently compared with free cells. In this study, the effects of entrapment on growth rate, metabolic activity, cell morphology and surface property, genetic material quantity and stress, and microbial diversity of bacterial cultures commonly found in wastewater treatment systems were investigated. Both pure and mixed cultures were studied. The type of entrapment media including no entrapment (free cells) and cell-to-matrix ratio were the main variables. Results indicated that cells entrapped in polyvinyl alcohol (PVA) had the highest specific growth and specific substrate utilization rates in most cases while cells entrapped in alginate provided the lowest rates for both mixed and pure cultures. The specific growth and specific substrate utilization rates were also affected by cell-to-matrix ratio and decreased as cell-to-matrix ratio increased. A cell morphological change from bacilli to coccoidal was observed in the case of alginate entrapment while the PVA entrapped cells had decreased diameter. The entrapment increased or decreased the surface roughness of cells depending on the type of entrapment media. A limitation of oxygen within the matrix caused the formation of putative nanowires only in the PVA entrapped cells. The alginate entrapment increased the deoxyribonucleic acid content of cells whereas the ribonucleic acid content of cells was increased by the PVA entrapment. The entrapment also created a stress condition to the cells as a consequence of the oxygen limitation. The carrageenan entrapment changed the bacterial community structure more than the alginate and PVA entrapments. The change in the bacterial community structure of the alginate entrapped cells was less time dependent than that of the PVA entrapped cells. The entrapment had similar effects on growth rate, metabolic activity, cell morphology and surface property, and genetic material quantity of major groups of bacteria in wastewater treatment. Among the media studied, PVA was the most promising choice for wastewater treatment applications because of high structural integrity and superior performances of entrapped cells associated with it.